Commercial distribution of electrical power is accomplished through multiphase AC networks which, in the United States, have been standardized as three phase networks. The economical distribution and extraction of powr from these networks relies upon proper phase matching between the lines.
Numerous safety devices have been developed which sense faults as a phase mismatch function and automatically open the network circuits. These devices are commonly referred to as network protectors which automatically disconnect elements associated with the network upon occurrence of a malfunction. The malfunction causing the automatic disconnect may be the result of or indicated by a phase mismatch between lines and the classical maintenance approach to determining the existence of such mismatches is for a maintenance person to enter the danger zone around the high voltage/current network protector. This zone is generally in a ground cavity where water buildup around the protector is inevitable or in a fenced area located outside so that typically maintenance personnel will be standing in a puddle of water or an another highly conductive surface while checking the system. Being thus precariously exposed, maintenance personnel must open the network protector case and measure the differences between phases by holding test prods at various points within the device, an action which requires placing hands in dangerous proximity to the high voltage/current lines.
Numerous attepmts have been made to create phasing or status indicators for power distribution systems but they are generally complex devices lacking reliability without fail-safe redundancies and in many instances require maintenance personnel to open the protective casings with greater frequency than with another device. For instance, J. Nudelmont, U.S. Pat. No. 3,778,801 on "Apparatus For Indicating Voltage Presence, Phase Sequence, Voltage Magnitude, Ground Faults, Etc. In An Electrical Power Distribution System" issued Dec. 11, 1973 discloses a monitoring circuit capable of indicating phase mismatch through indicator lamps. However, there is no redundancy within the circuit so an operator is not sure whether or not a mismatch indication is a fault within the network or test system. Furthermore, the reference fails to teach the concept of incorporating the circuitry within existing network protectors due to the complexity of the circuitry and the requirement for operator interaction with the test system which would necessitate opening the equipment case to manipulate the various control systems required to perform a test.
W. Murrills, U.S. Pat. No. 3,156,868 on "Phase Sequence Indicator Including Double Throw Switching Means" issued Nov. 10, 1964 is another example of utilizing lamp means to determine phase relationship between lines. Note that this system also requires manual switchingn operations to be performed by maintenance personnel and lacks fail-safe redundancy.
Phase related tests are required for maintenance troubleshooting and for verification of proper line connection before hinging a section of the network on line after maintenance or initial installation. The necessity for performing such tests on the high voltage/current lines is one of the primary factors leading to a relatively high mortality rate due to job oriented accidents for power distribution network maintenance personnel.